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Development of R290 transport refrigeration system (Part 1)

Development of R290 transport refrigeration system (Part 1)

Daniel Colbourne presented the following paper at last year’s Gustav Lorentzen Natural Working Fluids Conference in Edinburgh, Scotland, elaborating on the groundbreaking R290 project currently being trialled locally by Transfrig in association with the GIZ.

Transport refrigeration systems for small and large trucks have been developed in South Africa using R290. They are electrically driven via a diesel engine and an alternator and include a variable speed drive for capacity control. For the design of the systems, the main aspects addressed were circuit optimisation and integration of safety measures. Circuit optimisation comprised system simulations to select compressors and redesign of heat exchangers in order to achieve the same cooling capacities as R404A, whilst maximising improvement in coefficient of performance (COP) and reducing refrigerant charge as much as possible.

Subsequent to this, measurements were carried out to validate the performance with R290. To mitigate the flammability risk of using R290, several aspects were addressed.

In terms of the equipment redesign, other than charge size reduction, the main changes were to remove potential sources of ignition or apply pre-ventilation to remove any build-up of potentially flammable mixtures.

Fig1-CLA

Additionally, a leak identification feature was integrated into the system controls whereby a suspected substantial leak would result in a shutdown of the system and a warning signal to ensure no additional refrigerant can leak into the refrigerated space.

Extensive leak simulation tests were carried out to characterise the development of potentially flammable concentrations around the condensing unit and surrounding area, within the refrigerated and adjacent spaces. Conformity to the relevant parts of EN 378 and the essential health and safety requirements of the Atex (equipment) directive were confirmed.

Why do we need an alternative?
Transport refrigeration includes intermodal containers, refrigerated ships, refrigerated train carriages, air cargo containers, and refrigerated road vehicles, including vans, trucks and trailers. The emissions of refrigerant from this sector accounts for about 5% of the total (as tCO2-eq) although it varies by country. Due to their widespread use, refrigerated road vehicles (RRVs) represent the largest portion of direct emissions.

Typically, annual leakage is high compared to other subsectors, with values ranging around 15% to 50% of the system charge per year depending upon region, manufacturer and local conditions. Currently, the majority of RRVs use R404A and R22, with a smaller percentage on R134a and R410A. Although, recently some RRV system manufacturers announced their intention to use other alternative refrigerants, including R452A as well as R744.

In parallel to conventional vapour compression systems, several manufacturers are supplying open cryogenic systems with R728, where the refrigerant is vented to atmosphere. To date, there have been limited trials using hydrocarbons, particularly R290 in Australia, Germany and the UK. However, they do not seem to have become commercialised on a wider scale.

This article describes the development of a prototype R290 system for use in a medium-sized RRV. The starting point for the development was a baseline R404A system and it involved a number of stages.

Since the primary concern is overcoming the flammability hazard of R290, the majority of the steps address risk minimisation. This involved reduction of refrigerant charge, development of a leak control and safety system, improving leak tightness, identification of potentially flammable zones and subsequently addressing potential sources of ignition within those zones.

If the failure modes and effects analysis (FMEA) and quantitative risk assessment (QRA) yield any concerns, prior steps are re-evaluated in order to help minimise the risk further; these steps are consistent with the approach given in EN 1127-1. When the risk is believed to be sufficiently low, final compliance against a safety standard is carried out followed by drafting of guidance for users and finally applying the prototypes for field trials. In addition to safety matters, performance (capacity and efficiency) is also important. Since these parameters are closely linked to the charge reduction process, they were carried out simultaneously.

Baseline model description
The baseline model from which the prototype was developed is a factory sealed, pre-charged monoblock system that is installed into refrigerated trucks at the factory. The system comprises an evaporator, condenser, electronic expansion valve, compressor, receiver, accumulator, and interconnecting piping and valves. The compressor is electric driven so the condensing unit housing also comprises a diesel engine and alternator.

The nominal cooling capacity is approximately 8kW for medium temperature (MT) and 4.5kW for low temperature (LT) operation, which typically corresponds to truck bodies of around 7–9m in length. Originally, the refrigerant charge was 3.5kg of R404A and about 1.5kg when charged with R290.

Critical to consideration of safety measures, the numerous conditions that the system will experience must be accounted for. These include the RRV being in-use or not; the RRV being stationary (closed or un/loading) or in transit; it may be empty, loaded or partially loaded; the refrigeration system may be on, off or in (hot gas) defrost.

Charge reduction and performance optimisation
The first step in risk reduction is minimisation of the mass of flammable material, which may be partly achieved through charge reduction. The strategies for reducing refrigerant charge are well reported, although the optimal approach can vary depending upon the particular system architecture, operating conditions, and so on. As a first step, the strategy was as follows:

  • Select R290 compressor to provide at least the same nominal cooling capacity at rating conditions as the baseline R404A model.
  • Reduce charge in the condenser, evaporator, interconnecting piping, receiver, and accumulator as much as possible, whilst maintaining nominal cooling capacity and COP at rating conditions no lower than baseline R404A model and without negatively impacting upon system functionality.
  • Further optimise condenser and evaporator design to maximise COP without reduction in capacity.

The simulation tool IMST-ART (www.imst-art.com) was used to facilitate the charge minimisation and optimisation process. Performance testing the baseline model with R404A and R290 yielded data points against which the IMST-ART tool was calibrated and from thereon, the iterative charge minimisation and optimisation process was carried out with IMST-ART only.

 

 Acknowledgements

  The authors would like to acknowledge the German Ministry for Environment, Nature Conservation, Building and Nuclear Safety and the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH and Transfrig Ltd for giving permission for publication.

The key changes to the component design were: reduction of condenser and evaporator tube size (from 10mm to 5mm and 10mm to 7mm, respectively) and adjustment of their circuitry, smaller liquid line diameter and approximately halving the volume of the liquid receiver and accumulator. The redesign of heat exchangers was within the physical constraints of the existing coil block sizes.

A practical hindrance to the use of the optimum circuitry was the limited availability of heat exchanger production with small diameter tubes, so a compromise option was used. The finalised design yielded a working charge of 0.60–0.65kg R290, which is about 20% of the R404A charge size and 40% of the R290 charge in the baseline model.

Following confirmation of the model output with the R290 prototype, its performance over a range of ambient conditions is compared against that of the baseline R404A model for MT and LT, respectively. For both MT and LT conditions, the R290 model has consistently better COP (about 15–25% at MT and 10–30% at LT) and in fact at the lowest and highest ambient temperatures it tends to exhibit a greater improvement over R404A than at the rating conditions.

Fig3 4-CLA

Cooling capacity of the R290 model shows a “flatter” performance curve, i.e., the variation across a wide range of temperature is less than with R404A. At higher ambient temperatures the cooling capacity is greater than R404A (15-20%), whereas at lower ambient temperatures R290 tends to be about 5% lower than R404A; this behaviour is contributed to by the small diameter tubes in the R290 model leading to greater condenser pressure drop at higher capacity conditions. A broader parametric assessment indicated that greater improvement in COP could be achieved with the R290 model with alternate heat exchanger designs, but these required greater refrigerant charge and thus at the expense of increase in flammability risk.

Read Part 2 of this article

 

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